Work module for connection to a main conveyor line in a system of conveyor lines and a conveyor unit comprising an endless chain

ABSTRACT

A work module for connection to a main conveyor line in a conveyor system, in which the work module comprises a first transverse conveyor line, a longitudinal conveyor line and a second transverse conveyor line. The first transverse conveyor line is arranged to transport goods delivered from the main conveyor line to a longitudinal conveyor line in the module. The second transverse conveyor line is arranged to transport goods delivered from the longitudinal conveyor line to the main conveyor line. Further, each of the first transverse, longitudinal and second transverse conveyor lines in the work module comprises an outer conveyor line and an inner conveyor line.

TECHNICAL FIELD

The present invention relates to a work module for connection to a mainconveyor line in a system of conveyor lines. The present invention alsorelates to a conveyor or transport unit comprising a stand whichsupports an endless chain or belt, which is carried on a slide bar.

BACKGROUND ART

Within the manufacturing industry, pallet based work methods are common.In these cases, mounting and/or processing take place at a number ofwork stations along a process flow. Since mounting and/or processingtake place with a stationary product the possibility to supply a numberof work stations with products for processing is ensured, in spite ofthe fact that the products are stationary at a work station, by means ofarranging independent side transport flows to a main flow, whichcontinuously may feed products to and from said work stations, at whichside transport flows products may be removed from the main flow in orderto reach a work station and also be reintroduced to the main flow fromthe work station.

A realization of said conveyor system with a main flow and side flowsconsists of module based conveyor systems where an optional number ofwork modules, comprising a first and a second transverse conveyor lineand a longitudinal conveyor line, are connected to a main conveyor line.The first transverse conveyor line is linked to the main conveyor linevia a transfer station where goods or pallets may be moved from the mainconveyor line to the first transverse conveyor line. The secondtransverse conveyor line is linked to the main conveyor line via atransfer station where goods or pallets may be moved from the secondtransverse conveyor line to the main conveyor line. The first and secondtransverse conveyor lines are connected via transfer stations to thelongitudinal conveyor line. By means of its design, the work moduleadmits transport from the main conveyor line to a work station arrangedalong the longitudinal conveyor line. Examples of module based conveyorsystems are described in U.S. Pat. Nos. 5,947,259 and 5,884,746.

Module based conveyor systems comprising work modules according to thedescription above, are used in the manufacturing industry for electronicproducts, such as, for example, mobile phones, printed circuit cardsetc., which are sensitive for too fast electric discharges and inducedelectric currents Such products are usually called electrostaticdischarge sensitive or ESDS. A deeper description of the problem withelectrostatic discharges is for example provided in C62.38-1994 IEEE,“Guide to Electrostatic Discharge (ESD)” or via www.esda.org.

Induced electric currents may occur at the electronic product when theelectronic product is transported past a charged object. A too fastdischarge may take place If the electronic product gets into touch witha conducting material which is connected to ground. Said discharges orinduced electric charges may thus damage the electronic product. Inorder to reduce these problems, dissipative materials are used. Fordefinition of dissipative materials, it is referred to Swedish StandardSS-EN-61340-5. Examples of dissipative materials are provided in EP 434011 on page 2 and forward, and in U.S. Pat. No. 6,140,405 below“Background Art”.

A dissipative material sees to that a discharge of an electricallycharged object takes place slowly. This means that the magnitude of thecurrent at the discharge is limited, thus preventing damage onelectronic products when contacting the dissipative material. It isfurther seen to that the electronic products are discharged by means ofcontact with a dissipative material. When only electrically insulatingmaterials are used, there is a risk that an induced current arises atthe electronic product if it is transported through an electric fieldgenerated by an object placed along any of the conveyor lines where theelectronic product is travelling. The dissipative material also sees tothat other objects, except the electronic products, which are placed onthe dissipative material are discharged.

According to previously known technology, dissipative materials are usedat work modules solely within a limited work area situated behind thelongitudinal conveyor. The electronic product that will be processed,controlled or handled in any other way, is placed on the dissipativesurface, after which processing or control will take place. Since only alimited area within the work module is covered with a dissipativematerial, problems remain, since an operator not easily may distinguishbetween dissipatively protected areas and not protected areas. Further,according to previously known technology, the conveyors are notdissipatively protected, which means that possibly protected areas maynot be broken through by conveyor lines. This means that there is a riskthat the operator places the electronic products outside the work areawith the dissipative material, on an electrically conducting material,risking the consequence of a too fast discharge of the product. Further,there is a risk that the operator places electrically charged objects,for example tools of different kinds, on an electrically insulatedsupport in the vicinity of the work area. These objects will then not bedischarged, why there is a risk that that electronic components may bedamaged by the electric field that surrounds the charged object. Toolsmay also be electrically charged when they are placed outside the workarea if the tool gets in contact with an electrically charged area. Whena charged tool later is used in order to treat the electroniccomponents, there is a large possibility that the tool damages theelectronic product that shall be treated.

DISCLOSURE OF INVENTION

The object of the invention is to provide a work module for connectionto a main conveyor line in a conveyor system, where the risk of placingsensitive electronic products on an electrically conductive orelectrically insulating support is reduced. This object is accomplishedby means of a work module for connection to a main conveyor line in aconveyor, in which the work module comprises a first transverse conveyorline, a longitudinal conveyor line and a second transverse conveyorline, where the first transverse conveyor line is arranged to transportgoods delivered from the main conveyor line to the longitudinal conveyorline, and the second transverse conveyor line is arranged to transportgoods delivered from the longitudinal conveyor line to the main conveyorline, characterized in that the conveyor lines that are a part of thework module are equipped with a dissipative protection layer between theconveyor line and an underlying electrically conductive structure whichsupports the conveyor line.

As the conveyor lines that are a part of said work module are equippedwith a dissipatively conducting protection layer between the conveyorline and an underlying electrically conductive structure which supportsthe conveyor line, the creation of work modules with large protectedareas which include the conveyor lines is made possible. The presence oflarge protected areas reduces the risk of misplacing goods on aconductive support with a too fast discharge as result or,alternatively, misplacing on an electrically insulating support with nodischarge as result. Further, by using large protected areas, the riskof misplacing the tool outside a dissipative work surface arranged onthe work module is reduced, thus reducing the risk of the tool gettinginto contact with an area with a potential separate from the intended.

Further, by means of the invention, it is seen to that transported goodsautomatically gets into contact with a dissipatively conductingprotected area since the transport line is in contact with anelectrically conductive structure via a dissipative material. Byarranging transported electronic components on pallets, which offerelectrical contact with the support, it is seen to that discharge ofpossibly present electric charge of the components takes place in acontrolled manner in the work module.

In a preferred embodiment of the invention, the work module is formedwith an upper surface, comprising an area which is limited by the outerconveyor lines of the first and the second transverse conveyor line andthe longitudinal conveyor line, where a dissipatively conductingprotective layer essentially covers said area. Essentially covering hererefers to that at least 95% of the points which are possible to reachwith a ball with a diameter of 1 cm are equipped with a dissipativeprotective layer placed between said points and an underlyingelectrically conductive structure. The demand for being able to bereached by a ball with a diameter of 1 cm causes that that parts of theupper surface which only are possible to reach with a very thin probe donot have to be dissipatively protected. For example, parts of theelectrically conductive structure would probably be able to reach with avery thin probe at the conveyor belts. By means of the definition it isseen to that the upper surface essentially is covered with adissipatively conducting protective layer for practical purposes.

By means of the preferred embodiment, a possibility is admitted toachieve a dissipatively conducting protective layer for the whole workmodule except a minority of points within said area which preferably areformed with a higher surface resistivity than the dissipativelyconducting layer, thus reducing the risk of misplacing goods with toofast discharging or no discharging, respectively, as the consequence. Byallowing that 5% of the points are not covered with a dissipativelyconducting protective layer, a possibility is admitted to create worksurfaces which support, by way of example, electrical switches,lead-throughs for wires or sensors without these objects having to bedissipatively protected. It is of course desirable that all points aredissipatively conducting protected.

A second object of the invention to provide a dissipatively conductingprotected conveyor unit. Conveyor unit here refers to a stand, whichsupports an endless chain or belt carried on a slide bar. This object isaccomplished by means of a conveyor unit according to the characterizingpart of claim 14.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be described more in detail below with reference tothe appended drawings, where

FIG. 1 shows a work module in a top plane view,

FIG. 2 shows a work module seen in a front plane view along theintersection line II-II in FIG. 1,

FIG. 3 shows a chain driven conveyor line in detail,

FIG. 4 shows a cross-section along the intersection line I-I in FIG. 3,and

FIG. 5 shows an embodiment of a conveyor unit.

MODES FOR CARRYING OUT THE INVENTION

In FIG. 1, a conveyor system is shown comprising a work module 1 and amain conveyor line 2. The main conveyor line 2 consists of two parallelmain conveyor lines 3, 4 of which each one is formed as one or moreendless belts or chains. In the embodiment shown, the main conveyor line2 is made in three units, where a first and a second unit 5, 7 areequipped with each transfer station 8, 9 and also an Intermediate unit 6without transfer station. In the embodiment shown, the transfer stations8, 9 consist of transverse endless conveyor lines which are arranged ina parallel manner and designed to be vertically adjustable in adirection vertical to a plane through the upper surfaces of the mainconveyor lines 3, 4. In their raised position, the transfer stations arecapable of transporting goods from and to the work module, respectively,while in their lowered position transport of goods along the mainconveyor lines 3, 4 is enabled without coming into contact with thetransfer stations 8, 9. In an alternative embodiment, it is of courseconceivable to design the two parallel conveyor lines of the mainconveyor line to be vertically adjustable instead. The two parallelconveyor lines 3, 4 of the main conveyor line are preferably formed fortransporting goods in a predetermined direction, which is indicated withthe arrow 10. For re-transporting goods, another lower conveyor line(not shown) is thus preferably arranged, which transports goods in theopposite direction. The lower conveyor line is, in contrast to the mainconveyor lines 3, 4, preferably designed as one single return conveyorfine comprising two parallel conveyor lines of which each one is made asone or more endless belts or chains. For transferring goods between themain conveyor line and the lower conveyor line, there are lifts 11, 12which are arranged at each end of the main conveyor line 2. Each one ofthe lifts 11, 12 comprises conveyor lines 13-16 which are reversible fortransport both in the direction in to the elevator and out from theelevator.

The work module 1 comprises a first transverse conveyor line 20, asecond transverse conveyor line 21 and a longitudinal conveyor line 22.Conveyor line here refers to the means which executes the physicaltransport of the object which is placed on the conveyor line. These mayfor example consist of endless belts or chains or rotatable cylindersjournalled in bearings. Transverse here refers to that the conveyor linehas a transport direction which forms an angle with the transportdirection of the main conveyor line. Preferably the transverse conveyorline is arranged to be perpendicular in relation to the main conveyorline. Longitudinal here refers to that the conveyor line has a transportdirection which mainly is parallel to the transport direction of themain conveyor line. In the embodiment shown, the first transverseconveyor line 20 consists of two parallel conveyor lines, an inner line23 and an outer line 24. Outer line here refers to that the line liescloser to the edge of the work module than the inner line. According toa preferred embodiment, the inner and outer line are each formed as anendless conveyor line which are made with the same lengths. In the sameway, the second transverse conveyor line 21 consists of two parallelconveyor lines, an inner line 25 and an outer line 26. Further, thelongitudinal conveyor line 22 consists of two parallel conveyor lines,an inner line 27 and an outer line 28. The first transverse conveyorline 20 is designed to mainly transport goods from the main conveyorline 2 to a work station 29 situated along the longitudinal conveyorline 22, while the other transverse conveyor line 21 is designed tomainly transport goods from the work station 29 to the main conveyorline 2. The longitudinal conveyor line 22 is designed to transport goodsfrom the first transverse conveyor line 20 to the second transverseconveyor line 21 via the work station 29. However, the conveyor linesmay, of course, be designed to admit transport in both directions. Thearrows 30, 31 and 32 indicate the flow direction for the goods accordingto the preferred embodiment.

In order to admit transfer of goods from the first transverse conveyorline 20 to the longitudinal conveyor line 22, a first transfer station32 is arranged at a transfer area 33. The transfer area 33 is situatedwhere the first transverse conveyor line 20 intersects the longitudinalconveyor line 22. The first transfer station 32 admits transfer of goodsfrom the first transverse conveyor line 20 to the longitudinal conveyorline 22 as the first transverse conveyor line 20 is arranged to take afirst raised position when transport with the first transverse conveyorline 20 to said transfer area 33 is admitted without collision with thelongitudinal conveyor line 22 and a second lowered position wheretransport from said transfer area 33 with the longitudinal conveyor line22 is admitted.

According to a preferred embodiment, the first transverse conveyor line20 is designed with a first end 34 which is intended to be placed closeby said main conveyor line 2 without extending into the main conveyorline and a second end 35 which extends into the transfer area 33.

According to a further preferred embodiment, the first end 34 ispivotally arranged around a pivot axis 36 whereby the other end 36 isarranged to be vertically adjustable as the first transverse conveyorline 20 is rotated about its pivot axis 36. According to a preferredembodiment, the rotation is achieved by means of a driving motor 37 thatrotates an eccentrically arranged cam disk 38 on which a frame structure(not shown) for the first transverse conveyor line 20 rests.

Moving of goods between the main conveyor line 2 and each one of thetransverse conveyor lines 20, 21 takes place via the transfer station 8,9 mentioned above. These transfer stations preferably consists ofvertically adjustable endless conveyor lines situated between the twoparallel conveyor lines 3, 4 of the main conveyor line. The conveyorlines of the main conveyor line are thus uninterrupted at the transferstation. When moving goods from the main conveyor line 2 to the firsttransverse conveyor line 20, goods is moved over that conveyor line 4 ofthe main conveyor 2 which faces the work module 1. The goods is thenreceived by the first end 34 of the first transverse conveyor line.

According to a further preferred embodiment, the first transverseconveyor line 20 comprises two endless conveyor lines 23, 24 which arearranged in a parallel manner, where the endless conveyor lines are bentaround a centre point at said first end 34 of said first transverseconveyor line 20. According to this embodiment, the pivot axis 36 runsthrough the centre point, thus admitting rotation around the pivot axiswithout changing the vertical position for the transverse conveyor lineat the transition between main conveyor line and transverse conveyorline. The endless conveyor lines 23, 24 each consists of an endlesschain which, at respective end of the transverse conveyor line, issupported by a cogwheel or a carriage wheel 40. The pivot axis 36 iscommon with a centre axis which supports the cog-wheels or carriagewheels 39 at the first end 34.

In a preferred embodiment, the cog-wheels or carriage wheels 39 at thefirst end 34 are freely journalled in bearings.

The second end 35 of the first transverse conveyor line 20 is situatedin connection with, but inside, the outer conveyor line 28 of thelongitudinal conveyor line 22, which consists of a driven endlessconveyor line 41 in the transfer area 33. According to a preferredembodiment, within said transfer area, the inner conveyor line 27 of thelongitudinal conveyor line consists of a passive unit 42 situatedbetween the inner and outer conveyor lines 23, 24 of the firsttransverse conveyor line. Passive unit here refers to a part of aconveyor line that is not driven. When a conveyor line is formed of twoparallel lines, it is possible to make one of these lines as a passiveunit. The other line that is arranged to be parallel sees to that goodsis moved along the line. Passive units should not be too long, astransported goods may be rotated as a consequence of that the goods isnot symmetrically driven The passive unit 42 preferably consists of aset of cylinders which are not driven, journalled in bearings.

In order to admit movement of goods from the longitudinal conveyor line22 to the second transverse conveyor line 21, a second transfer station50 is arranged in a transfer area 51 where the second transverseconveyor line 21 intersects the longitudinal conveyor line 22. Thesecond transfer station 50 admits transfer of goods from thelongitudinal conveyor line 22 to the second transverse conveyor line 21as the second transverse conveyor line 21 is arranged to take a firstraised position when transport with the second transverse conveyor line21 out from said transfer area 51 is admitted without collision with thelongitudinal conveyor line 22, and a second lowered position wheretransport with the longitudinal conveyor line 22 to said transfer area51 is admitted. According to a preferred embodiment, the secondtransverse conveyor line 21 is designed with a first end 52 which isintended to be placed in connection with said main conveyor line 2 and asecond end 53 which extends into the transfer area 51. By means of thisdesign, the main conveyor line 2 may be formed as on single continuesline comprising two parallel main conveyor lines 3, 4 of which each oneis formed as an endless belt or chain without the transverse conveyorline overlapping the main conveyor line. According to a furtherpreferred embodiment, the first end 53 is pivotally arranged around apivot axis 54 where the other end is arranged to be verticallyadjustable as the second transverse conveyor line 21 is rotated aboutits pivot axis 54. According to a preferred embodiment, the rotation isachieved by means of a driving motor 55 that rotates an eccentricallyarranged cam disk 56 on which a frame structure (not shown) for thesecond transverse conveyor line 21 rests.

The second transverse conveyor line 21 comprises two endless conveyorlines 25, 26 which are arranged in a parallel manner, where the endlessconveyor lines are bent around a centre point at said first end 52 ofsaid second transverse conveyor line 21. According to this embodiment,the pivot axis 54 runs through the centre point, thus admitting rotationaround the pivot axis without changing the vertical position for thetransverse conveyor line at the transition between main conveyor lineand transverse conveyor line. The endless conveyor lines 25, 26 eachconsists of an endless chain which, at respective end of the transverseconveyor line, is supported by a cog-wheel or a carriage wheel 57, 58.The pivot axis 54 is common with a centre axis which supports thecog-wheels or carriage wheels 57 at the first end 52.

In a preferred embodiment, the cog-wheels or carriage wheels 57 at thefirst end 52 are freely journalled in bearings.

The second end 53 of the second transverse conveyor line 21 is situatedin connection with, but inside, the outer conveyor line 28 of thelongitudinal conveyor line 22, which consists of a driven endlessconveyor line 43 in the transfer area 51. According to a preferredembodiment, within said transfer area, the inner conveyor line 27 of thelongitudinal conveyor line consists of a passive unit 59 situatedbetween the inner and outer conveyor lines 25, 26 of the secondtransverse conveyor line. The passive unit 59 preferably consists of aset of cylinders which are not driven, journalled in bearings.

According to a preferred embodiment of the invention, the outer conveyorline 28 of the longitudinal conveyor line 22 is formed as a passive unit60 in an area 61 between the inner conveyor lines 23, 25 of the firstand second transverse conveyor lines 20, 21. The area extends in adirection along the longitudinal conveyor line 22 that at leastcomprises the extension of the work station in this direction.Preferably, the passive unit has an extension which exceeds 40 cm insaid direction. The passive unit 60 is preferably made of a set ofcylinders which are journalled in bearings. Within this area, the goodsis transported by means of the inner conveyor line 27 of thelongitudinal conveyor line 22, which within the area 29 consists of oneor more driven endless lines 62.

FIG. 2 shows the conveyor system in FIG. 1 seen in a front plane viewalong the cut line II-II. In the figure, the left-hand part of an edgeside of the upper plane surface of the work module 76 has been cut awayin order to more clear show lines which are a part of the work module.The conveyor system comprises a work module 1 arranged in connectionwith a main conveyor line. The conveyor system is made with a lowerconveyor line 70, which is made as a single main conveyor linecomprising two parallel conveyor lines of which each one is made as anendless belt or chain. For transferring goods between the upper and thelower line, there are lifts 11, 12 which are arranged at each end of themain conveyor line 2. Each one of the lifts 11, 12 comprises conveyorlines 13-16 which are reversible for transport both in the direction into the elevator and out from the elevator. In the figure, the conveyorline 14 that belongs to the lift 11 is shown in its lower position fortransfer of goods between the lower conveyor line 70 and the lift, andalso the conveyor line 16 in its upper position for transfer of goodsbetween the lift and the main conveyor line 2.

Further, a stand 71 is arranged, which supports the work module 1. Onthe left-hand side of the work module shown in the figure, the drivenendless conveyor line 41, which constitutes the outer conveyor line 28of the longitudinal conveyor line 22 in the transfer area 33, is shown.Further, the inner conveyor line 23 and the outer conveyor line 24 ofthe first transverse conveyor line 20 is shown in its lowered position.The first transverse conveyor line 20 is supported by a frame structure72 according to what has been stated above, where the verticaladjustment of the first transverse conveyor line is achieved by means ofa driving motor 37 that rotates an eccentrically arranged cam disk 38.In the embodiment example shown, the driving motor is supported by a rod73 which is fastened to the stand 71.

On the right-hand side of the work module shown in the figure, thedriven endless conveyor line 42 which constitutes the outer conveyorline 28 of the longitudinal conveyor line 22 in the transfer area 51 isshown. Further, the inner conveyor line 25 and the outer conveyor line26 of the second transverse conveyor line 21 is shown in its raisedposition. The second transverse conveyor line 21 is supported by a framestructure (not shown) according to what has been stated above, where thevertical adjustment of the second transverse conveyor line is achievedby means of a driving motor 55 that rotates an eccentrically arrangedcam disk 56. In the embodiment example shown, the driving motor issupported by a rod 74 which is fastened to the stand 71.

Further a driven endless line 62 is shown, which constitutes the innerconveyor line 27 of the longitudinal conveyor line 22 in the area 29.The driven endless line 62 is situated behind the passive unit 60 of theouter line 28 of the longitudinal conveyor line 22. Further, the edgeside 75 of the upper plane surface 76 of the work module, whichconstitutes a work surface, is shown. The edge side 75 may be formed asshown in FIG. 2 with an area 77 which corresponds to the area 29 of thework station where the edge is equipped with a lesser vertical extensionthan at the module's left (not shown) and right part 78.

In a preferred embodiment, the work module 1 is formed with an uppersurface comprising an area 112 which is limited by the outer conveyorlines 24, 26, 28 of the first and the second transverse conveyor line20, 21 and also the longitudinal conveyor line 22. The area is furtherpreferably limited by the rear edge of the upper surface 76, which isarranged to face the main conveyor line. When the rear edge is placed ata sufficiently large distance from the front edge, in what way anoperator not accidentally may touch the rear edge, the protected areamay be limited to what normally is within reach for an operator in thedirection towards the main conveyor. In this case, an area surroundingthe rear edge does not have to be protected. Said area is essentiallycovered with a dissipatively conducting protective layer.

Dissipatively conducting here refers to that the surface has a surfaceresistivity between 10⁴ and 10¹⁰ Ω/sq. The surface shall be measured at23° C. and at a humidity of the air of 12% RH.

“Essentially covered” here refers to at least 95% of the points whichare possible to reach with a ball with a diameter of 1 cm are equippedwith a dissipatively conducting protective layer placed between saidpoints and an underlying electrically conductive structure. According toa preferred embodiment, the dissipatively conducting protective layer ofthe upper surface 76 within said area (112), except at those areas wherethe included conveyor lines (20, 21, 22) run, consists of a plastic matwhich is applied on a supporting surface. The plastic mat iselectrically connected with the underlying conductive structure 71. Theplastic mat may be made in an optional dissipatively conductingmaterial, for example such that has been disclosed in EP 434 011 andU.S. Pat. No. 6,140,405 and previously known materials also disclosedthere.

The underlying electrically conductive structure may consist of a stand71 which supports the upper surface 76 and also a stand 91 (FIG. 4)which supports the lines. According to a first embodiment, the pointswhich are not covered with a dissipatively conducting protective layerhave a resistance to ground exceeding 1.0*10⁸ Ω and admit a discharge ofan electrostatic charge exceeding 100 V faster than 2 s. According to analternative embodiment, the whole upper surface is covered with adissipatively conducting material within the area 112, except atlead-throughs in the material where conveyor lines 20, 21, 22 that arepart of the work module 1 run.

In a further preferred embodiment, the whole upper surface is equippedwith a dissipatively conducting protective layer placed between saidpoints and an underlying electrically conductive structure 71, 92, 93.

According to a preferred embodiment, the resistance between thedissipatively conducting surface and the underlying structure lieswithin the interval 7.5*10⁵ and 1.0*10⁸ Ω.

All conveyor lines within the area 112 are equipped with a dissipativelyconducting protective layer between line and ground.

In a further preferred embodiment, the endless conveyor lines 23, 24,25, 26, 27, 41 and 42 which are a part of the module 1, are made bychains which are supported by slide bars, where the chains are made ofor coated with a conductive material and said slide bars are made of orcoated with a dissipatively conducting material.

Alternatively, the chains may be made of a dissipatively conductingmaterial. Such an embodiment is not advantageous, since dissipativelyconducting materials usually are expensive and the consumption ofmaterial is larger than when sliding ledges made in a dissipativelyconducting material are used. For the passive units, the cylinders thatare journalled in bearings which are included, are preferably coatedwith or made of a dissipatively conducting material. Alternatively, thecylinders may be journalled in bearings via or be suspended in astructure made of or coated with a dissipative material.

According to what has been described above, the work module 1 issupported by the stand 71 The work module is thus formed as a workingplace with a space for the legs 81 arranged below the work station 29 Inorder to provide a good work environment, it is important to minimizethe distance between the upper surface 76 and the roof 80 of the spacefor the legs, at the front edge of the work station. According to apreferred embodiment of the invention described above, the outerconveyor line 28 of said longitudinal conveyor line 22 is for thispurpose designed as a passive unit 60. As the passive unit may be madewith a very small, or none at all, mounting height, if the passive unitconsists of a slide line, said distance may be reduced considerablycompared with the case where a driven unit is used. According to apreferred embodiment, the work module 1 has a distance between a plane79 which touches upon the upper surface of the passive unit and the roof80 of a space for the legs 81 below said passive unit 60 which fallsbelow 8 cm. In a preferred embodiment said distance falls below 4 cm.

FIG. 3 shows a side view of a chain driven conveyor unit 90. A conveyorunit here refers to a unit which supports a conveyor line in the form ofan endless belt or chain. The conveyor unit comprises a stand 91 whichaccording an embodiment is made by two aluminum profiles 92, 93 (FIG. 4)which are joined together. The aluminum profiles are made in a waypreviously known for anyone skilled in the art and are arranged tosupport slide bars 94, 95 and wheel axles 96, 97 for cogwheels 98, 99that are driven or freely journalled in bearings. Together with theslide bars 94, 95, the cogwheels or carriage wheels 98, 99 support anendless chain 100 which is joined by means of a set of links 101 whichare coupled to each other in a previously known manner. The cogwheels orcarriage wheels support a gear ring 102 which meshes the correspondingrecesses in the chain 100.

In the example shown, the stand consists of two angled aluminiumprofiles 92, 93 which together form a square profile which partlysupports said slide ledges 94, 95, partly supports the wheel axles 96,97 for the cogwheels or carriage wheels 98, 99. In a preferredembodiment of the invention, at least one of the cogwheels or carriagewheels 98, 99, which are arranged in the bending areas of the chain, isfreely journalled in bearings. When both the cogwheels or carriagewheels are freely journalled in bearings, at least a third cogwheel 105is arranged, which is driven by its wheel axle 106, for example. bymeans of a cogwheel 107 that is mounted on the outside of the stand 91and accessible for a drive unit.

As apparent from FIG. 4, the slide ledges 94, 95 are preferably madewith vertically extending protrusions 108, 109 which extend a distancepast the side walls 110, 111 of the chain 100. The outgrowths 108, 109control conveyed objects and prevent lateral movement of these.

In order to provide a sufficient protection against damage on objectsfrom partly too fast discharges of electrostatic charge, partly damagedue to electric fields originating from movements of electrostaticallycharged objects, the chain 100 is made of or coated with a conductivematerial and the slide ledge is made of or coated with a dissipativelyconducting material.

According to an embodiment of the invention which is shown in FIG. 5,the conveyor unit is made as described in the patent document SE 516849or WO 02/02440. In this document, a slide rail arrangement for aconveyor unit is described, which has a drive device in a conveyor linedriven endless conveyor chain. The conveyor chain is supported andcontrolled on upper and lower flanges (111, 111′ (not shown), 112, 112′(not shown)) running in the beam direction, on at least one profile beam(110), which flanges are arranged from the corresponding profile sidestowards the profile centre. The conveyor unit is equipped with at leastone pair of deflecting devices in order to conduct the conveyor chain(120) and reverse the movement direction of the chain from a firstdirection to the opposite from a corresponding pair of slide rails (131;127) on the upper flanges.

According to a preferred embodiment, each one of the slide rails has apart (A) with a essentially U-shaped cross-section extending in thedirection of the slide rail. Further, one of the shanks (133, 133 a) ofsaid part has a longitudinal part (B) made in one piece, that togetherwith said shank forms a locking lug that extends in the longitudinaldirection of the slide rail. The corresponding flange (111, 111′ (notshown), 112, 112′ (not shown)) on the profile beam has a recess (136,136′) that extends in the longitudinal direction of the flange with astop face (134′, 134″) that prevents the locking lug (134, 134A) and theslide rail (131, 127) from being displaced towards the profile beamcentre plane.

The description of a conveyor unit according to the patent document SE516849 or WO 02/02440 are completely incorporated in this application.

The inventions shall not be limited to the embodiments described above,but may vary freely within the scope of the appended claims.

1. A work module for connection to a main conveyor line in a system ofconveyor lines, said work module comprising: a first transverse conveyorline; a longitudinal conveyor line; a second transverse conveyor line;each of said conveyor lines comprising an outer conveyor line and aninner conveyor line; an upper surface, said upper surface having an areawhich is limited by said outer conveyor lines of said first transverse,second transverse and the longitudinal conveyor lines; a dissipativelyconducting material comprising a plastic mat essentially covering saidupper surface within said area except at positions within said areawhere said first transverse, second transverse and longitudinal conveyorlines run; and an underlying electrically conductive structuresupporting each of said conveyor lines, said first transverse conveyorline being arranged to transport goods delivered from said main conveyorline to the longitudinal conveyor line, said second transverse conveyorline being arranged to transport goods delivered from said longitudinalconveyor line to said main conveyor line, in each of said firsttransverse, longitudinal and second transverse conveyor lines areequipped with a dissipatively conducting protection layer between saideach conveyor line and said underlying electrically conductivestructure, and said dissipatively conducting protection layer has asurface resistivity in the range between 10⁴ Ω/sq and 10¹⁰ Ω/sq.
 2. Atransport unit comprising: a stand; an endless chain or belt; and atleast one slide bar, said stand supports said endless chain or beltwhich is carried on said slide bar, and supports a first and a secondcogwheel or carriage wheel, which are arranged in a respective bendingarea of said chain or belt, said first and second cogwheels or carriagewheels are made of dissipatively conducting material, said slide bar ismade of or coated with a dissipatively conducting material and saidchain or belt is made of or coated with a conductive material, and saiddissipatively conducting material has a surface resistivity in the rangebetween 10⁴ Ω/sq and 10¹⁰ Ω/sq.